1. Field of the Invention
This invention relates to gas turbine combustion systems and in particular to such systems comprising a plurality of combustion chambers, hereinafter referred to as combustors.
2. Description of Related Art
The combustion system in a gas turbine plant commonly comprises a number of combustors arranged in a parallel array in a common air flow, at least some of the combustors being ignited in series. On start-up, one or more of the combustors are ignited and the flame is spread to the other combustors via interconnecting tubes, the pressure difference between the interconnected combustors causing the flame to spread. A typical arrangement is shown in FIG. 1, in which three combustors 1, 2, 3 are interconnected by tubes 4. Normally, of course, there would be more combustors, typically six or eight connected in a closed ring.
One of the life-limiting problems associated with this ignition technique is the damage caused to the tubes, or the combustors to which they are attached, by the flow of hot gases between combustors during normal running after light-up. Successful air cooling of the interconnecting tubes tends to be difficult because cooling air bled into them also has the effect of reducing the cross-lighting performance. It may also cause hot combustion gases to be carried by the air flow between combustors.
Existing designs for the interconnecting tubes depend for their operation on effusion cooling, impingement cooling or film cooling.
Effusion cooling utilises an array of small diameter closely pitched cooling holes spread over the tube wall surface. Each hole bleeds a jet of cooling air through the wall but with very little penetration, so that a cooling barrier is formed. This method tend to be inefficient in its utilisation of air and may give either reduced cross-lighting performance or insufficient cooling.
Impingement cooling involves the use of double skin walls for the tube so that cooling air may be injected through an array of holes in an outer tube to impinge forcibly on an inner tube and so cool it. The cooling air is thus constrained to flow in the gap between the inner and outer tubes. A disadvantage of this method is its mechanical complexity, particularly when applied to small components.
Film cooling, in which a cooling air flow is inlet at one end of the interconnecting tube and directed along and in contact with the inner wall of the tube, tends to induced an `enjector mechanism` whereby hot gases from one combustor are carried along with the cooling air flow towards the other combustor. This ejector mechanism continues in normal running conditions, i.e. when the pressure difference between the two combustors has been substantially reduced, because the cooling air flow tends to carry hot combustion gases with it, continuously heating up the tube upstream of the air entry point with detrimental effect. The result is that the interconnecting tubes become cracked or burnt and need regular replacement.